[Haskell-cafe] Read Instance for UArray won't port to linux

[Björn Bringert]

Simon Peyton-Jones wrote:
> | The error in linux is:
> |     Illegal instance declaration for `Read (UArray Int Double)'
> |         (The instance type must be of form (T a b c)
> |          where T is not a synonym, and a,b,c are distinct type variables)
> |     In the instance declaration for `Read (UArray Int Double)'
> |
> | Why does it want three parameters for the instance type?  I am baffled by
> | this.
> 
> It's saying that in Haskell 98 you can have
>         instance Read (UArray a b)
> (for type variables a, b) but not
>         instance Read (UArray Int Double)
> 
> (As Spencer says, use -fglasgow-exts to lift the H98 restriction.)
> 
> However, you mis-read the error message to say "the instance type must have three parameters", which isn't what I meant at all!  I was trying to use an example of the general form, but conveyed the wrong idea.
> 
> I can see why you read it that way.  Now you understand, can you give me a better error message?

I'm not the original poster, but what about just changing it to "The 
instance type must be of the form (T a1 ... an) where T is not a 
synonym, and a1 ... an are distinct type variables)"?

/Björn